The present invention relates to a polymer composition, in particular a composition comprising a base polymer which is a poly(alkyl(meth)acrylate) and a rubber toughening agent.
Base polymer such as a poly(alkyl(meth)acrylate), e.g. poly(methyl methacrylate), known as PMMA, can be a relatively brittle material. This detracts from its use in certain applications, e.g. external light clusters for automotive vehicles, where properties such as impact resistance are important. Conventionally, the properties of the base polymer can be improved by the addition of comonomers during its production and/or by the addition of toughening agents. Typically, the toughening agents are small particles having a core-shell structure, e.g. as disclosed in GB-A-2039496. The toughening agents are added as pre-made particles which are then compounded with the base polymer. Such toughening agents are prepared using emulsion phase polymerisation in which each layer is sequentially polymerised one on another. Consequently, due to the complexity of the core-shell manufacturing process, the base polymer so toughened is relatively expensive and is primarily confined for use in specialised applications.
Certain rubber toughening agents are used to improve the properties of polystyrene. EP-0818481-A2 describes a process for the manufacture of high impact polystyrene (HIPS) materials using low shear mechanical agitation such that the product produced contains large particle sizes of a rubber-like composite phase up to 12 micron for shear rates all lower than 50 rpm. These particle sizes are retained when this polymeric mixture is further extruded into an article.
It has now been found that base polymer such as a poly(alkyl(meth)acrylate) as previously described can be toughened by the addition of certain rubber toughening agents which are added during the production of the base polymer. The polymer composition so formed has improved impact resistance compared to that of the base polymer and has a unique morphology.
Accordingly, in a first aspect, the present invention provides a polymer composition comprising
(a) a base polymer which is a poly(alkyl(meth)acrylate); and
(b) a rubber toughening agent selected from the group consisting of
(i) copolymers of (alkyl)acrylate rubbers
(ii) copolymers of vinyl aromatic monomers with (alkyl)acrylates
(iii) copolymers of vinyl aromatic monomers with conjugated diolefins
(iv) copolymers of vinyl aromatic monomers with monoethylenically unsaturated monomers
wherein the rubber toughening agent is present in the polymer composition as discrete islands substantially surrounded by the base polymer and in which islands are sub-islands of the base polymer.
In a second aspect the present invention provides a process for the preparation of a polymer composition comprising
(a) a base polymer which is a poly(alkyl(meth)acrylate); and
(b) a rubber toughening agent selected from the group consisting of
(i) copolymers of (alkyl)acrylate rubbers
(ii) copolymers of vinyl aromatic monomers with (alkyl)acrylates
(iii) copolymers of vinyl aromatic monomers with conjugated diolefins
(iv) copolymers of vinyl aromatic monomers with monoethylenically unsaturated monomers
wherein the rubber toughening agent is present in the polymer composition as discrete islands substantially surrounded by the base polymer and in which islands are sub-islands of the base polymer which process comprises dissolving the rubber toughening agent in a monomer composition from which the base polymer is to be formed to form a solution, polymerising the monomer composition to form the base polymer and thereby causing the rubber toughening agent to come out of solution so as to form islands within the polymer composition in which are included sub-islands of the base polymer.
In a third aspect the present invention provides a moulded article formed from a polymer composition comprising
(a) a base polymer which is a poly(alkyl(meth)acrylate); and
(b) a rubber toughening agent selected from the group consisting of
(i) copolymers of (alkyl)acrylate rubbers
(ii) copolymers of vinyl aromatic monomers with (alkyl)acrylates
(iii) copolymers of vinyl aromatic monomers with conjugated diolefins
(iv) copolymers of vinyl aromatic monomers with monoethylenically unsaturated monomers
wherein the rubber toughening agent is present in the polymer composition as discrete islands substantially surrounded by the base polymer and in which islands are sub-islands of the base polymer.
For the rubber toughening agent (i) copolymers of (alkyl)acrylate rubbers and (ii) copolymers of vinyl aromatic monomers with (alkyl)acrylates include both saturated and unsaturated copolymers.
The base polymer may be a homopolymer or a copolymer. Preferably the base polymer contains an alkyl methacrylate, such as a C1-C4 alkyl methacrylate, e.g. methyl methacrylate. Particularly preferred is when the base polymer is a copolymer of such an alkyl methacrylate and an alkyl acrylate, such as a C1-C4 alkyl acrylate, e.g. ethyl acrylate.
The rubber toughening agent may be selected from a wide variety of commercially available and other copolymers. Suitable copolymers of vinyl aromatic monomers with (alkyl)acrylates include styrene-butyl acrylate (St-BA) and styrene-lauryl methacrylate (St-LMA) rubbers, preferably incorporating allyl unsaturation. Examples of monomers that can provide allyl unsaturation include allyl methacrylate (AMA) and glycidyl methacrylate (GMA) Suitable copolymers of (alkyl)acrylate rubbers include benzyl methacrylate-lauryl methacrylate (BzMA-LMA) rubber also preferably incorporating allyl unsaturation. Suitable copolymers of vinyl aromatic monomers with conjugated diolefins include styrene-butadiene rubbers such as Nipol NS210., polystyrene-polybutadiene block copolymers such as Cariflex 1184, S1509, S1013. Suitable copolymers of vinyl aromatic monomers with monoethylenically unsaturated monomers include styrene ethylene butylene styrene (SEBS) copolymer.
It is preferred that the rubber toughening agent contains at least some degree of unsaturation, for example where the rubber toughening agent is a styrene-butylacrylate rubber it is preferred that the level of allyl unsaturation is from 1.5 to 5.0% (on a molar basis), particularly from 1.5 to 4.5%, and especially about 2%. Where the rubber toughening agent is selected from saturated rubbers such as styrene-butyl acrylate (St-BA), styrene-lauryl methacrylate (St-LMA) and benzyl methacrylate-lauryl methacrylate (BzMA-LMA) rubbers we find that less of the rubber toughening agent is present in the desired island/sub-island morphology whilst a proportion is present in the form of discreet, separate phase.
The islands of rubber toughening agent are generally circular in cross section and typically up to very large average particle sizes of 250 microns or less in diameter, for example from 10 to 100 microns in diameter. However, where the optical properties of the polymer composition are especially important small diameter islands, e.g. from 2 to 5 microns, are desirable. Therefore it is surprising that when an unsaturated styrene-laurylmethacrylate rubber is used even with islands of up to 10 microns in size the optical properties of the polymer composition remain acceptable whereas a comparable polymer composition containing an unsaturated styrene-butylacrylate rubber is translucent and hazy.
Typically, the rubber toughening agent represents from 1 to 20% preferably from 1 to 10% by weight of the polymer composition. More preferably, the rubber toughening agent represents from 2 to 6% by weight of the polymer composition and in particular about 4%, for example 96% by weight of a base polymer containing methyl methacrylate and with 4% by weight of a styrene-butadiene rubber.
The polymer composition is prepared by dissolving the rubber toughening agent in a monomer composition from which the base polymer is to be formed, for example a mixture of methyl methacrylate and ethyl acrylate, so as to form a solution. The preparation of the solution may be assisted with stirring or agitation and is usually conducted at room temperature. Polymerisation of the monomer composition is then carried out using conventional bulk polymerisation techniques, for example by the addition of suitable initiators such as lauroyl peroxide and chain transfer agents such as lauryl mercaptan. Preferably the temperature under which the polymerisation is conducted is controlled. Additionally, at least during the initial stages of the polymerisation, e.g. up until about 50%, and preferably up until about 35%, of the monomer composition has been polymerised, a high level of shear may be applied, for example between 100 and 250 rpm, for example about 170 rpm. However it is preferred that the polymerisation of the monomer composition is undertaken without stirring.
Alternative polymerisation routes, e.g. by solvent, are not known to give rise to the desired morphology.
The polymer compositions of the present invention can be compression moulded or processed in other conventional ways to form articles having improved impact resistance. In such articles the desired morphology is still apparent although the size of the islands may change. Surprisingly, in some cases, it was found that the particle size of the islands of rubber toughening agent were greatly reduced when the polymer compositions were further processed, for example extruded or injection moulded. For example average particle sizes were found to be reduced from tens of microns to less than 5, particularly 0.01 to 3 micron and especially 0.1 to 1 micron, for example 0.1 to 0.5 micron. Even more surprisingly the extruded or injection moulded articles were found to have properties at least equivalent to the corresponding polymer compositions with much larger particle sizes. In some instances the properties of these moulded articles were found to be enhanced as compared to the polymer compositions.
The polymer compositions as herein described may also contain conventional additives including other modifiers such as the previously described core-shell particles which may then be added at lower than usual levels to achieve the same improved properties.
The present invention is illustrated by reference to the following diagram.
FIG. 1 is a Transmission Electron Micrograph (TEM) of a section through a polymer composition of PMMA (96%) with styrene-butadiene (4%) which has been produced with shearing at 170 rpm during the initial stage of polymerisation and which shows the desired morphology. The base polymer (1) has discrete islands of rubber toughening agent (2) which contain sub-islands of base polymer (3).
FIG. 2 is a TEM of a section through a polymer composition of the same composition as used in FIG. 1 but which has not been sheared during the initial stage of polymerisation.